Plant growth and development are controlled by intrinsic growth regulators or hormones and environmental cues through interconnected signal transduction pathways. A single hormone can regulate many different processes and likewise different hormones can cooperate to control the same cellular process. Plant steroid hormones, brassinosteroids (BRs), regulate many growth and developmental processes such as cell elongation, senescence, vascular development, reproduction, and various stress responses. BRs signal through a membrane-localized, leucine-rich repeat (LRR) receptor kinase BRI1 and co-receptor BAK1 to regulate several intermediate signaling components. BR signaling eventually controls BES1 and BZR1 family transcription factors, which mediate the expression of many genes for various BR responses. Consistent with the main function of BRs in cell elongation, BRs promote the expression of a number of cell wall remodeling enzymes required for cell elongation. In addition, BRs also regulate a variety of other genes implicated in signal transduction including several receptor-like kinases (RLKs), which are postulated to be involved in communicating BR signals to other pathways.
Receptor like kinases (RLKs) are involved in transmission of extracellular signals into the cell. The RLK proteins have a modular structure, starting from the N-terminus with a secretion signal that gets processed, an extracellular domain, a single transmembrane domain and a cytoplasmic kinase domain. Receptor like kinases are postulated to form homodimers or heterodimers of two related kinases, similar as for animal receptor kinases (Torii, Curr. Opin. Plant Biol. 3, 361-367, 2000). Animal receptor-like kinases mostly have tyrosine kinase activity, whereas plant RLKs all have Ser/Thr kinase specificity, or may sometimes have a dual specificity. In animals, most of the RLKs act as growth factor receptors, whereas plant receptor like kinases may function in various processes, including development, hormone perception or pathogen responses. An overview of developmental functions of plant receptor like kinases such as meristem development, pollen-pistil interactions, hormone signaling, gametophyte development, cell morphogenesis and differentiation, organ shape, organ abscission and somatic embryogenesis is given by Becraft (Annu Rev. Cell Dev. Biol., 18, 163-192, 2002).
Receptor-like kinases may be grouped according to the structure of their extracellular domain (Shiu and Bleecker, Proc. Natl. Acad. Sci. USA 98, 10763-10768, 2001). The largest group was that of the Leucine Rich Repeat (LRR) containing RLKs; which may be split up in 13 subgroups (LRR I to LRR XIII) based on the organization of the LRR domains in the extracellular part of the RLK. The LRR units may be present in varying numbers and may be arranged in continuous or interrupted repeats.
WO 2004/007712 describes and characterizes a number of Arabidopsis RKS genes that encode LRR receptor-like kinases. It was postulated that modification of expression of the RKS genes would cause a modification of the brassinosteroid-signaling pathway. The data show that, depending on the specific RKS gene and the kind of expression (up- or downregulation of expression compared to wild type), results in various phenotypes. For example, RKS4 and RKS10 are reported to stimulate cell division. Overexpression of the RKS4 gene resulted in increased cell division and an altered plant phenotype, whereas modulation of RKS10 did change the cell number, but not plant or organ size. Overexpression of RKS10 also caused the formation of many generative meristems that did not end up in normally developed flowers. Both overexpression and down-regulated expression of RKS10 had a strong negative effect on pollen formation. Root length was negatively affected by overexpressing RKS10, while initiation and outgrowth of lateral roots was promoted. The same effects on root growth may be obtained by suppressing RKS1 expression. Also overexpression of the RKS3, RKS4 or RKS6 genes had positive effects on root length. Increased apical shoot meristem formation and outgrowth was obtained by overexpressing RKS0 but also by downregulating expression of RKS3, RKS4, RKS8 or RKS10. RKS4 overexpression was reported to result in larger seed size, but did not result in higher seed yield; no functional analysis was made of the RKS11 gene.
Accordingly, the ability to regulate the brassinosteroid induced pathways, and concomitant receptor like kinases, to influence many different agricultural traits of interest, is of considerable value to commercial agriculture. The present invention provides new mechanisms for plant growth, which are influenced somewhat by the brassinosteroid pathway, but also represent a separate and independent pathway involving several receptor like kinases that influences plant growth, cellular elongation and other yield related traits. These and other features will become apparent from the description of the invention which follows.